'Male specific' H-Y antigen of the mouse cross-reacts serologically with a cell surface component fund in the heterogametic (XY) sex of all vertebrate species so far tested including man. This widespread phylogenetic conservation led to the proposal that H-Y is the product of the mammalian testis-determining gene. To evaluate that proposal we studied H-Y in human subjects with anomalous sex chromosome constitutions, including patients with structural modifications of the Y-chromosome and patients whose chromosomal or gonadal sex do not correspond with their secondary sex phenotype. H-Y gene loci were mapped to the pericentric (male-determining) region of the Y short arm, and testicular differentiation was invariably associated with presence of H-Y (and normal ovarian differentiation with its absence). As human subjects provide by far the most favorable material for this kind of investigation, we propose to continue our study of H-Y in human subjects with sex chromosome anomalies in order to: 1. resolve whether there may be one or several H-Y genes on the human Y-chromosome and whether (if there are several) these comprise a family of structural determinants, 2. clarify the role of H-Y gene products alone and in conjunction with other cell surface components such as HLA in in normal and abnormal sex determination, and 3. develop and evaluate new methods of H-Y typing (hemagglutination, fluorescence, cytotoxicity, mixed hemadsorption) as diagnostic measures in the clinical detection of Y-chromosomal genes, with possible therapeutic applications.